Determining How to Clear a Virus Infection from the Brain Without Harming Brain Cells

Dorian McGavern, Ph.D.

The Scripps Research Institute

Funded in January, 2004: $300000 for 4 years

Determining How to Clear a Virus Infection from the Brain Without Harming Brain Cells

The researchers will use a mouse model of a viral brain infection to determine how immune "memory" T cells can successfully clear the virus from infected brain cells, without harming the brain cells themselves. If researchers can define the minimum immunological conditions necessary to achieve this feat in experimental animals, the research would facilitate the development of "immunocytotherapies" to safely rid infected humans of potentially deadly brain viruses.

The brain is a relatively "immune privileged" site that possesses mechanisms to dampen immune defenses. Scientists speculate that this situation may have evolved to protect the brain from the potentially damaging effects of a severe immune response. In the brain, the generation of new replacement cells is minimal compared to cell regeneration in the rest of the body. Consequently, this trade-off may have been evolutionarily necessary to preserve brain functioning.

As a result, however, a number of viruses such as HIV, herpes, measles, and adenovirus manage to invade and infect the brain, without being attacked by the immune system. Interestingly, the investigators have discovered in an animal model that immune cells taught to attack a specific virus can be infused into the animal, and subsequently eliminate the virus without killing the brain cells that the virus has infected. These investigators now will determine how the immune T cells accomplish this extraordinary task. This would be an important step toward development of a new therapeutic strategy to treat viral brain infections in humans.

The investigators will use mice infected from birth with lymphocytic choriomeningitis virus (LCMV). This virus affects humans as well as mice, and has the potential to cause deadly brain meningitis. The immune systems of the mice infected from birth, however, do not recognize the virus as foreign and do not attack it. In contrast, mice that are infected with the virus sometime after birth develop an immune response to the virus. In these mice, certain immune cells, called "memory" T cells, learn to recognize the virus. These memory T cells then remember what the virus looks like and attack it whenever it returns. The researchers have infused these "memory" T cells into the brains of the mice that are infected with the virus from birth. This process is called "adoptive transfer." The researchers found that the adoptively transferred memory T cells eliminated the virus from the animals brains without killing the brain cells harboring the virus. This took 100 days and 357,000 memory T cells, but the investigators do not know how the memory T cells actually accomplished this feat.

Now, the investigators will conduct five studies to learn what occurs between the adoptively transferred memory T cells and the brain cells that harbor the virus. They will use high-resolution microscopy to determine how the infected brain cells send a signal about their plight. Next, the investigators will pinpoint movements and locations of the memory T cells, to establish when the maximal interactions occur between the memory T cells and virus-infected brain cells. Then the researchers will evaluate the interactions between the immune T cells and the virally-infected brain cells at these times. Since the T cell's membrane undergoes substantial reorganization when it engages with a virally infected cell, the researchers will be able to quantify the number and characterize the nature of the T cells' interactions with the infected brain cells.

Thereafter, the investigators will identify the specific chemicals (called cytokines) that immune cells release to kill the virus, and determine whether the T cells directly release these cytokines or instruct other immune cells to release them. Finally, the researchers will determine whether two types of memory T cells, which target different regions of the viral protein, are sufficient to rid the brain of the virus via this adoptive transfer technique.

Determining How to Clear a Virus Infection from the Brain Without Harming Brain Cells

The central nervous system (CNS), which harbors a population of nonreplicative cells (i.e., neurons), is one of the most complex tissue compartments in the entire body. The immune system is an equally sophisticated collection of tissues capable of mobilizing a cellular defense that can specifically eliminate target cells containing an ill-favored foreign invader. Normally the interplay between the immune system and the pathogen proceeds without incident (i.e., the pathogen is successfully eliminated from the host). However, if the pathogen gains access to the CNS, the immune system is faced with a new series of challenges. Because the CNS is essential for survival, evolutionary pressures have likely led to the acquisition of several "immune dampening" mechanisms that abate the ability of the immune system to perform maximally. Consequently, a broad array of pathogens seek refuge within the immunologically hampered CNS, which can result in neurobehavioral abnormalities, neurologic dysfunction, and, in extreme circumstances, death. Persistent infection of the CNS has a global impact on human health, and thus, it is of the utmost importance to investigate (and ultimately harness) strategies to relieve individuals of these unwieldy infections.

The proposed study will attempt to gain novel mechanistic insights into a remarkable therapeutic strategy referred to as immunocytotherapy. Adoptive transfer of memory T lymphocytes (both CD8+ and CD4+) into a host bearing the burden of a persistent CNS viral infection from birth results in pathogen clearance in the absence of cellular damage. Contemporary T cell visualization strategies will be employed to simultaneously monitor the coordinated activities of antigen-specific CD8+ and CD4+ T cells as well as their interactions within the CNS during immunocytotherapy (a hitherto unexplored area of research). The ultimate goal of this research is to operationally define the minimum requirements for successful pathogen clearance following immunocytotherapy, in order to determine how best to translate this approach into a clinical setting.

Dorian McGavern, Ph.D.

Dorian McGavern, Ph.D., is an Assistant Professor in the Department of Neuropharmacology at The Scripps Research Institute. He obtained his bachelor's degree from the University of Pennsylvania in 1995 and then continued on to the Mayo Clinic, where he obtained a Ph.D. in molecular neuroscience in 2000. Following the completion of his graduate work, he pursued post-doctoral training at The Scripps Research Institute under the direction of Dr. Michael Oldstone, one of the world's leading authorities in viral immunology.

Dr. McGavern has since accepted a faculty position at The Scripps Research Institute, and his present research group remains dedicated to fully understanding the parameters that underlie successful clearance of a persistent viral infection from the brain and spinal cord (i.e., the central nervous system, or CNS). This is accomplished by utilizing novel visualization strategies to follow the activities and interactions of immune cells as they engage virus-infected targets residing in the CNS. It is anticipated that the definition of such parameters will enable us to develop strategies to relieve humans of unwieldy persistent CNS infections (e.g., HIV-1, herpes virus, measles virus, etc).

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